This invention relates to a control circuit for controlling the starting of a gas discharge lamp powered by an electronic ballast. More specifically, it relates to controlling the pre-heat time of a gas discharge lamp by holding off the full striking voltage until the filaments have had sufficient time to pre-heat.
Many present day gas discharge lamps are powered by electronic ballasts which operate the lamp at a frequency above 25 kHz to obtain higher efficiency than what is possible with 60 Hz operation. Many electronic ballasts consist of a rectifier to convert 60 Hz AC to DC, a boost circuit to increase the DC voltage and achieve power factor correction, and an inverter to convert the DC to high frequency AC.
Several pre-heating techniques are currently known in the prior art. Many present day ballasts preheat the lamp filaments with a voltage supplied from the inverter before the boost circuit comes on. Once the boost circuit starts, the voltage at the ballast output terminals will rise to a level sufficient to strike the lamp arc. An example of this technique is shown in U.S. Pat. No. 5,650,925 titled "Diode Clamping Arrangement for Use in Electronic Ballasts." This technique is not applicable in ballast circuits that do not use active power factor correction.
Another pre-heating technique is to use frequency shifting. This technique is illustrated in German laid open patent application DE 3208607 titled, "Ballast for at least one load which is periodically ignited and powered by a generator." It is also shown in U.S. Pat. No. 4,935,669 titled, "Two mode electronic ballast." These patents show a ballast circuit that operates at a high initial frequency that is approximately double the full load switching frequency of the resonant inverter output circuit. During this initial high frequency operation, the voltage developed in the resonant circuit is insufficient to light the lamp. After the lamp filaments are heated, the frequency is reduced to the full-load switching frequency and the voltage across the resonating inverter output circuit rises to cause the lamp arc to strike. This frequency shifting approach suffers from requiring a complicated circuit to adjust the operating frequency.
A third preheating technique utilizes a thermistor to reduce the ballast output voltage during preheating. This technique, shown in U.S. Pat. No. 4,647,817, has reduced efficiency because the thermistor dissipates substantial heat even after it has changed to a high-resistance state.
Several symmetry control circuits have been shown in the prior art. One is shown in U.S. Pat. No. 5,583,402 titled "Symmetry Control Circuit and Method." This circuit shows a symmetry control circuit which modulates the duty cycle of the lower transistor in a half-bridge configuration in order to dim the lamps. A second symmetry control circuit is shown in U.S. Pat. No. 4,983,887. This patent discloses using the symmetry control technique to modulate the duty cycle of the lower transistor in a half-bridge configuration in order to limit the open circuit voltage during ballast operation in order to prevent damage to components in the circuit. A third symmetry control circuit is shown in German patent DT 3338464 titled, "High Frequency Brightness Control Device for Fluorescent Lamps." This circuit shows a symmetry control circuit which modulates the duty cycle of the lower transistor in a half-bridge configuration in order to dim the lamps. It does not show controlling the ballast for pre-heating the lamp. None of these three patents teach using symmetry control for lamp pre-heating.
Another symmetry control circuit is shown by a ballast produced in February of 1994 in Italy by MagneTek S.p.A. This ballast employs a symmetry control technique to control the duty cycle of the lower transistor in a half-bridge inverter during start-up and operation. The inverter is self-oscillating. The schematic of this ballast shows a NPN transistor Q5 that is used to adjust the duty cycle of the lower power switch Q3 by turning Q3 off before the time when it would naturally turn off. A capacitor C3 is linked to the midpoint of the half bridge, and is discharged when Q3 turns on. A PNP transistor Q8 is biased to form a current source that is connected to capacitor C3, and begins to charge it after it is discharged. When C3 is sufficiently charged, a PNP transistor Q4 turns off, and NPN transistor Q5 turns on, which turns off transistor Q3, thereby limiting the duty cycle of transistor Q3. During preheating, the charging current provided by Q8 is held high by a timer circuit, and the duty cycle of Q3 is held low so that the ballast output voltage will be low. After the preheat interval, a regulator circuit controls the current provided by Q8, which adjusts the duty cycle of Q3 so that the lamp current can be maintained at the desired level. This ballast requires a large number of components, and is expensive to produce.
An unmet need currently exists in the field of ballast design for a simple, inexpensive and efficient circuit to provide filament heating, especially for ballasts without boost power factor correction. This need can be met with a circuit that shifts the symmetry of the square-wave inverter voltage during preheating to produce a low ballast output voltage while the filaments are being heated. After the filaments have had sufficient time to become heated, then the symmetry control is removed and the ballast output voltage rises to a level that allows the lamp to strike.